This invention relates to semiconductor laser devices, and more particularly to a semiconductor laser device which is low in threshold value and has an excellent temperature characteristic.
A variety of semiconductor laser devices of different structures have been known in the art. BC (buried crescent) type semiconductor laser devices and BH (buried helerostructure) type semiconductor laser devices are particularly known for their oscillation mode stability and low threshold values.
FIG. 1 is a sectional diagram outlining a conventional BC type semiconductor device. In FIG. 1, reference numeral 1 designates an n-InP semiconductor substrate having a first electrode 2 on its second major surface and having a carrier density of about 7.times.10.sup.8 /cm.sup.3 ; 3, a first semiconductor layer of p-InP which is formed on the first major surface of the semiconductor substrate by liquid phase epitaxial growth and has a carrier density of approximately 1.times.10.sup.18 /cm.sup.3 ; 4, a second semiconductor layer of n-InP which is formed on the first semiconductor layer 3 by liquid phase epitaxial growth; 5, a belt-shaped groove etched in from the surface of the second semiconductor layer to the semiconductor substrate 1; and 6, a fifth semiconductor layer of n-InP which is formed on the bottom of the groove 5, namely, the main surface of the substrate, by liquid phase epitaxial growth in such a manner that one portion thereof is in contact with the bottom of the groove 5 and another portion is in contact with the lower portion of the side 3a of the first semiconductor layer 3. A semiconductor layer 6 which is the same as the fifth semiconductor layer is formed on a part of the second semiconductor layer 4 as shown.
Further in FIG. 1, reference numeral 7 designates an active layer of n-InGaAsP of crescent section which is formed on the fifth semiconductor layer 6 in the groove 5 by liquid phase epitaxial growth in a manner such that both ends thereof are in contact with the side walls 3a of the first semiconductor layer 3. The active layer 7 is smaller in band gap than InP, forming a so-called double heterojunction. At the same time the active layer 7 is formed, an InGaAsP layer 8 is formed on the remaining parts of the fifth semiconductor 6. The active layer 7 may be of p-InGaAsP. Reference numeral 9 designates a third semiconductor layer of p-InP which is formed on the second semiconductor layer 4 and the InGaAsP layer 8 and on the active layer 7 in a manner such that it is in contact with the upper portions of the side 3a of the first semiconductor layer 3 and the side walls 4a of the second semiconductor layer 4. A second electrode 10 is formed on the major surface of the third semiconductor layer 9.
In FIG. 2 showing a BH semiconductor device, reference numeral 1 designates a semiconductor substrate of n-InP; 7, an active layer of n- or p-InGaAsP which is formed on a first major surface of the semiconductor substrate 1; 9, a third semiconductor layer of p-InP which is formed on the active layer 7 by liquid phase epitaxial growth; the upper portion of the substrate 1, the active layer 7 and the third semiconductor layer 9 being etched on either side so as to form a belt-shaped protrusion; and 3, first semiconductor layers of p-InP formed on both sides of the belt-shaped protrusion by liquid phase epitaxial growth in a manner such that the sides of the active layer 7 are in contact with inner sides 3a of the first semiconductor layers. Further in FIG. 2, reference numeral 4 designates second semiconductor layers of n-InP formed on the first semiconductor layers 3; 11, sixth semiconductor layers of p-InP formed on the second semiconductor layers 3; and 10, a second electrode formed on the third and sixth semiconductor layers 9 and 11.
In both the BC type semiconductor laser device and the BH type semiconductor laser device, current flows as described below. FIG. 3 shows the device of FIGS. 1 or 2 in the form of a model. The flow of current will be described with reference to FIG. 3.
The device is structurally designed so that, in order to improve the laser oscillation efficiency, current is caused to collectively flow in the active region 7, which generally has a small width, approximately 2 .mu.m, in view of oscillation mode control. In other words, the structure of the device is such that the second semiconductor layers 4 of a first conductivity type are provided in a second conductivity type layer consisting of the first and third semiconductor layers 3 and 9 (or the first, third and sixth semiconductor layers 3, 9 and 11 in FIG. 2), thus forming a so-called slit. Because of this structure, the second semiconductor layers 4 in the second conductivity type layers serve as an energy barrier against holes, which are charge carriers in the second conductivity type layers. Accordingly, under normal conditions, current cannot flow over the second semiconductor layers. That is, the second semiconductor layers 4 serve as a slit for reducing the amount of current, so that the current is caused to flow collectively in a small width of the active layer 7.
In the device thus constructed, a p-n junction 12 of InP is unavoidably formed between the semiconductor substrate 1 and possibly the fifth semiconductor layer 6 and on one side the first semiconductor layer 3 on the other side. Accordingly, the structure is such that a p-n-p-n layer consisting of the third semiconductor layer 9 (or the sixth semiconductor layers 11) of the second conductivity type, the second semiconductor layers 4 of the first conductivity type, the first semiconductor layers 3 of the second conductivity type and the semiconductor substrate 1 of the first conductivity type in the stated order from above is formed near the active layer 7. This structure is considerably similar to that of a thyristor. In the semiconductor substrate 1, the amount of added impurities is relatively large in order to decrease its defect density, and in general the carrier density is approximately 7.times.10.sup.18 /cm.sup.3. In the first semiconductor layer 3, the carrier density is, in general, low, not more than 2.times.10.sup.18 /cm.sup.3 (1.times.10.sup.18 /cm.sup.3 in the prior art), because the layer 3 is formed by liquid phase epitaxial growth. Therefore, the device is readily turned on as a thyristor due to a leakage current Ig as shown in FIG. 3.
The structure shown in FIG. 3 can be represented by the electrical equivalent circuit of FIG. 4. When the current I.sub.D flowing in the active layer 7 is small, the leakage current Ig is also small, and therefore the thyristor structure is not turned on or energized and the applied current effectively contributes to oscillation. When the temperature rises, the amount of current required for oscillation is increased and the applied current must be increased. However, if the leakage current Ig is also increased, the thyristor structure is turned on and a larger inactive current I.sub.S flows. Thus, the current I.sub.D in the active layer 7 is so small that the oscillation is ineffective and unstable.
FIG. 5 is a graphical representation showing the relationships between the current flowing between the first and second electrodes 2 and 10 and the output of the laser device with the ambient temperature changed. As is apparent from FIG. 5, at low temperatures the current contributes effectively to the laser output; however, at high temperatures the current vs Laser output characteristic exhibits saturation, so that the current does not effectively contribute to the laser output, and at worst the oscillation is stopped.